To find out how many more miles Oliver has to go to reach his grandparents' house, you can subtract the distance he has already traveled from the total distance.
Total distance to grandparents' house: 228.4 miles
Distance traveled: 137.65 miles
Distance remaining = Total distance - Distance traveled
Distance remaining = 228.4 miles - 137.65 miles
Distance remaining = 90.75 miles
So, Oliver has 90.75 miles left to go.